Wind turbines are used at various places in the world to generate electrical power from wind energy. In the United States, important areas where substantial numbers of wind turbines of various kinds have been installed and are now operating are in the Altamont area of California east of San Francisco Bay, in the vicinity of Tehachapi, Calif., and also in the vicinity of North Palm Springs, Calif., in the Coachella Valley proximate the southern foot of Banning Pass. In each of the three geographic areas noted above, the wind turbines are operated and maintained by various operators on a contract basis with the owners of the turbines. Both owners and operators of wind turbines desire that the turbines operate reliably with minimum maintenance and repairs of breakdowns.
A common style of wind turbine of Danish design and manufacture has a turbine rotor which is composed of three long slender blades mounted symmetrically about a hub. The hub is carried on the end of a rotor shaft mounted in a nacelle which is carried atop a supporting tower or pylon for rotation about a vertical yaw axis of the turbine. Within the nacelle, the rotor shaft is coupled through a gearbox to a generator which generates electrical power in response to rotation of the rotor, at a characteristic constant speed, by action of wind on the rotor. The coupling of the nacelle to its supporting tower includes a yaw drive mechanism which includes a large diameter main gear fixed to the tower concentric to the yaw axis and with which is engaged a yaw pinon gear. The pinion gear is driven by a yaw drive motor within the nacelle. Operation of the yaw drive motor is controlled by a wind direction sensor which operates the motor to turn the nacelle about its yaw axis to cause the rotor shaft to point into the prevailing wind direction. In such wind turbines, it has been found that the yaw drive mechanisms fail by breakage of the yaw pinion shafts, or by spalling or other failure of the teeth on the yaw pinion gears, beginning at about 3800 .+-. hours after the turbine has been placed in service. It has been determined that such yaw drive failures are caused, in the large majority of instances, by imbalance of the rotor assembly.
A typical rotor assembly for a wind turbine of the kind described above weighs about 4000 pounds. The hub weighs approximately 1000 pounds and each of the three blades mounted to the hub weighs about 1000 pounds. The total weight of the turbine nacelle including the rotor assembly is on the order of 12,000 pounds; there are some kinds of turbines now in use in which the nacelle, with its rotor, weighs 17,000 pounds or more.
The diameter of a rotor of the Danish design turbine is 52.5 feet. As manufactured, the individual blades for such a wind turbine are statically balanced. Such balancing is accomplished by supporting the opposite ends of the blade on respective scales, and noting the measured weight of the blade as so positioned. If the measured blade weights are not within certain limits, balancing weights of appropriate size are placed in a tube extending within the blade and are fixed at desired locations along the length of the blade to cause the measured end weights of the blade to fall within the desired limits. The blades for a given rotor are individually statically balanced as a set in this manner.
It is common that a set of blades, even though statically balanced to be essentially perfectly matched, manifest unbalance when mounted to the rotor and the rotor is turning at its service speed which, in the case of the Danish designed turbines mentioned above, is about 47 revolutions per minute or about 0.781 Hz., i.e., approximately 3/4 revolution per second. A wind turbine rotor assembly having three identically matched statically balanced blades can manifest unbalance when turning because of blade-to-blade variations in the distribution of the weight within the blades so that the effective rotating center of gravity of the entire rotor assembly is not on the axis of rotation of the rotor. Such imbalance has been discovered to be the principle cause of failure in the yaw drives of such turbines. Rotor imbalance also contributes meaningfully to the failure, and frequency of failure, of other components of a wind turbine.
In view of the size and weight of the rotor assemblies of wind turbines, it is apparent that the ability to balance a wind turbine rotor in place is very much preferred over balancing such a rotor at a location removed from the wind turbine site. Dismantling and transport of a wind turbine rotor is costly and time consuming; the blades, though large, are delicate, and the risk of damage to them increases the more they are handled.
Sophisticated equipment has been developed and is in wide use throughout the world for balancing rotating masses such as automobile wheel and tire sets, electric motor and electric generator armatures, to name but a few of the many kinds of rotating things which need to be balanced for efficient and reliable operation over extended periods. Those things, however, rotate at relatively high velocities in use as compared to wind turbine rotors which, as noted above, turn very slowly, namely, at less than one turn per second, in use. The existing methodology and apparatus developed for balancing rotating masses is geared to the balancing of masses which rotate at substantially higher revolutions per minute than do wind turbines. A straight forward application of that technology to the balancing of wind turbines has heretofore not been effective. Heretofore, the problem of balancing a wind turbine rotor turning at its service speed has been unsolvable.